New Zealand contribution to the hunt for MH370 is likely to exceed $2.5 million and become the air force's largest-ever peacetime search.

It's government policy not to divulge how much it is costing the New Zealand taxpayer to search for the missing Malaysia Airlines plane off the coast of Perth. No cutoff date for the search has yet been determined.

"Kiwi Rescue", the code-name for missions flown by a P3K Orion out of Perth, has turned into the Royal New Zealand Air Force's largest ever peacetime search.

Defence Minister Jonathan Coleman declined to provide its cost and the hours flown, but Kiwi Rescue is likely to be around 11 per cent of the total hours Orions can fly in a year. That amounts to around $2.5m.

The spending has implications across the Defence Force and for the duration of future searches with the Government already saying the military budget will not be increased.

Labour Party defence spokesman Phil Goff said there was no question New Zealand should be part of the MH370 search but people were entitled to know what it cost.

"It is good global citizenship, the plane has gone down and the families of all on board are worried about that [but] there is no reason why the Government should withhold from the public how much is being spent on the search," said Goff, who warned New Zealand not to hold its breath for reimbursement.

The Malaysia Airlines Boeing 777 disappeared on March 8 with 239 people aboard - including former Christchurch man Paul Weeks and Aucklander Ximin Wang - while flying from Kuala Lumpur to Beijing.

Within a week a New Zealand P3K Orion, a 48-year-old aircraft heavily upgraded with the latest surveillance equipment, was helping the search, first in the South China Sea.

New Zealand is one of only 19 countries that have ratified the 1979 International Convention on Maritime Search and Rescue, which requires countries to develop plans for the rescue of persons in distress at sea.

The convention divides the world's oceans into 13 search and rescue areas and imposes considerable obligations on parties - including paying the costs of searches.

New Zealand, with the world's largest single search and rescue zone, never discloses what searching costs, other than to say there is an annual budget of $177m for the Airborne Surveillance and Response Force which is mostly 5 Squadron and its six Orions.

The RNZAF rents out its services which gives a clue to the search costs: the Ministry of Primary Industries is billed $11,000 per hour in the air, plus fuel, for fisheries patrols.

Kym Bergmann, a former Australian Government adviser and now editor of Asia Pacific Defence Reporter, told the Sunday Star-Times, the $11,000 per hour figure "is certainly in the ballpark".

He based that on the Australian Orions which were upgraded in the 1990s. New Zealand's Orions are just completing their $352m upgrades so the hourly rates were likely to be higher.

Coleman's office would not give the most recent flight hours for Kiwi Rescue, but when they were being made public the plane had done 105 confirmed hours. In the 14 days since, based on media reports, the crews have probably completed 105 hours. That would suggest they are now at around 200 hours at a total estimated cost of $2.2m.

Then there is fuel. On current listed Jet A-1 prices, commercially it costs $29,000 to tank up an Orion. The air force has been saying it is flying 11-hour missions each time, pointing to a total fuel bill of $522,000. In normal times foreign military forces pay their own fuel bills.

Kiwi Rescue would also have other expenses including hotels and food to the mundane, but charged, cost of having the plane washed down to remove the salt after each mission.

The search for MH370 was costing the Australian Defence Force at least A$800,000 ($862,000) per day and Bergmann said if the cost grew it could cut in to other activities such as Operation Sovereign Borders, the federal government's policy of turning back asylum seeker boats that approach from Indonesia.

Last week Reuters estimated the 26 countries contributing planes, ships, submarines and satellites to the search had spent at least US$44m ($51m). That is about the same as the € 32m spent looking for Air France's Flight AF447, which crashed into the Mid-Atlantic in 2009. Australia has paid about half the cost.

There is no international agreement on how the costs of multinational search operations for aircraft should fall. The 1944 Convention on International Civil Aviation requires only that states secure "the highest practicable degree" of co-operation relating to "aircraft in distress and investigation of accidents".

LOOKING FOR THE LOST

While MH370 seems certain to be the biggest search ever, there have been similarly expensive global searches.

In some ways, people are still looking for Amelia Earhart and Fred Noonan who disappeared in the north Pacific in 1937.

The US$4 million ($76m in today's dollars) four-month search was the longest and most costly to that date. Nothing was ever found.

The idea of using satellite tracking evolved from the 1972 disappearance of two US congressmen in a plane crash in a remote area of Alaska. A 39-day search over 800,000 square kilometres (greater than the Indian Ocean search) failed to find anything.

A huge international search in 1983 was launched when a Korean Airlines Boeing 747 disappeared near Sakhalin with the loss of 269 people. It was less of a search for the Soviet Union – they knew where it was as they shot it down.

New Zealand has a history of extensive searching for wreckage In 2005 it took two weeks of intensive searching to find missing liquor millionaire Michael Erceg and Guss Klatte, whose helicopter crashed in bush near Raglan.

Extensive shipping searches included one in 1955 for the ship Joyita, sailing between Samoa and Tokelau. The boat eventually drifted to Fiji – but the 25 souls aboard disappeared with no trace.

In 1989 there was an extensive search for the missing yacht Rose Noelle with four men aboard. The search failed, but 119 days later the boat drifted ashore and the men survived.

Last year Maritime NZ conducted this country's largest search, for the seven people aboard the ketch Nina in the Tasman Sea, covering 2.4m square kilometres but finding nothing.